The present invention relates to an image forming apparatus of the type using an electrophotographic procedure and, more particularly, to a method of maintaining the density of images to be produced by such an apparatus to be adequate at all times.
With an electrophotographic copier, facsimile machine, laser beam printer or similar image forming apparatus, it is a common practice to control the toner concentration or density of a developer to an adequate level for the purpose of stabilizing the quality of a toner image to be formed on a photoconductive element or similar image carrier or to be transferred to a paper sheet. Various approachs have heretofore been proposed to implement such a control over the toner density. One of them is to form a toner image representative of a reference density pattern having a reference density on a photoconductive element, sense the density of the toner image by using a reflection type photosensor which is constituted by a light emitting and a light-sensitive element, compare the sensed density and a predetermined reference density, and then control the supply of toner in response to the result of the comparison. Another approach is such that a toner density sensor is disposed in a developing unit to sense the mixture ratio of toner and carrier of a developer in terms of inductance, whereby the supply of toner is controlled in response to a change in inductance. Further, the supply of toner may be controlled by sensing a change in a current which flows through a developer. All of the prior art methods have merits and demerits, as follows.
The photosensor scheme, for example, promotes accurate control over toner density because it directly senses the density of a developed toner image, i.e., the final image, optically. However, the light emitting and light-sensitive elements of the phtosensor are positioned so close to the surface of a photoconductive element that they are apt to be smeared by toner particles which are scattered around during operation. This causes the photosensor to malfunction and bring the toner density out of control. The approach implemented by a toner density sensor which is disposed in a developing unit cannot detect the toner density with accuracy because the sensor is susceptible to the influence of so-called spent toner, for example, which exists in a developer but does not contribute to development. Even if the toner density sensor may accurately sense toner density to allow toner supply to be controlled in response to its output, the potential of a latent image formed on a photoconductive element is effected by the deterioration of the element and charges as well as changes in ambient conditions with no regard to the toner density, so that the density of the final toner image is not always adequate.
The image density sensor and the toner density sensor discussed above may be combined to insure stable image density control at all times, as has also been proposed in the past. For example, in a control method disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 57-136667, the toner density sensor accommodated in a developing unit senses toner density of a developer, and the sensed toner density is compared with a predetermined reference value or toner sensing level. In response to the result of the comparison, the amount of toner supply is adjusted to control the toner density of the developer. On the other hand, the reflection type image density sensor located near the surface of a photoconductive element senses the toner density of a reference image, i.e., a toner image produced by developing a latent image of a reference image which is formed on the element in a certain pattern. Whether or not the density sensed by the image density sensor lies in a normal controllable range is then determined. If it lies in such a range, whether or not the sensed density of the toner image of the reference image is equal to an adequate level is determined. If the result of the decision is positive, a print mode operation is executed. If otherwise, whether the sensed density is higher or lower than the adequate level is determined. If the sensed density is higher than the adequate level, the above-mentioned reference value is switched to a smaller value; if otherwise, it is switched to a larger value. In this manner, the reference value serving as a predetermined reference level is varied on the basis of the density of the reference image sensed by the image density sensor.
Further, when the density of the toner image of the reference image sensed by the image density sensor does not lie in the controllable range, the prior art control method regards that the sensor is not operating properly due to one cause or another and decides to use the above-stated reference value as the toner density sensing level associated with the toner density sensor. Specifically, the toner density control is continued by using the reference value until the image density sensor recovers its function with the cause of the fault being cleared up. Should the reference value be changed in response to the toner density sensed by the image density sensor which is not normal, the new reference value or sensing level would become unusual. Controlling the toner density of the developer by adjusting the supply of toner in response to such a sensing level would naturally provide the toner with extraordinary density. If the toner density control is continued by using the reference value as a temporary measure as stated above, it is possible to use the apparatus efficiently in the event of the fault of the image density sensor because it is not necessary to interrupt the operation of the apparatus or the supply of toner. Of course, the control in such a condition is different from the ordinary image density control.
While the image density sensor does not operate properly as stated above, the prior art method uses the predetermined reference level as a density sensing level associated with the toner density of the reference image and controls the toner density based on the reference density. The toner density attainable with such a control is not accurate although accuracy will be restored in due course. Specifically, the reference level or sensing level is selected beforehand by taking account of the kind and characteristics of a developer to be used as well as the ambient conditions. Hence, the reference value selected beforehand will not always be optimal at the time of fault of the image density sensor due to the aging of the developer and the changes in ambient conditions which may occur before the fault. This will be seen from the fact that the prior art method does not fix the reference value, i.e., the reference value is variable. It follows that implementing the reference level by the predetermined reference level at the time of fault, even if it may be temporary, prevents the toner density from being controlled with accuracy.